Driving under the influence of alcohol (DUI) is one of the most commonly-committed crimes in the U.S.

DUI trials are so common that they’re usually the first trials that new prosecutors and defense attorneys are assigned to, because the procedure is usually pretty much the same. In almost every case, a key piece of evidence that the driver on trial was, in fact, drunk is their blood alcohol content (BAC) at the time of their arrest. This is most often determined through the use of a device that can test a person’s BAC in the field, usually a Breathalyzer or similar device.

However, there have always been questions about how accurate and reliable these testing devices are, especially if they are improperly used or maintained. For that reason, police departments have strict rules in place regarding how these devices are stored, maintained, and used. Of course, that doesn’t mean that these rules and procedures are always followed to the letter.

Case in point: hundreds of DUI convictions in San Francisco may have to be thrown out because some officers of the San Francisco Police Department may have improperly tested and maintained the equipment that’s used to conduct BAC tests.

Without going into the details of how the technology behind these devices works (because I don’t fully understand it, either), I’ll provide a basic overview: most devices that measure BAC via a person’s breath need to be calibrated every week or so. Otherwise, they become increasingly inaccurate as time passes. This calibration is done by using canisters of air that contain a known amount of alcohol vapor, which is applied to the testing device. By doing this, you can gauge how accurate the tester’s readings are, and then make any necessary adjustments.

There’s now concern that the police department did not conduct these accuracy checks properly, which means that hundreds of people may have gotten DUI convictions based on bad evidence.

This is a very serious issue, and it’s a bit like the fiasco of the San Francisco crime lab a few years ago. In that case, the drug lab of the SFPD found out that an employee had stolen drugs for their own personal use, and the subsequent investigation revealed widespread evidence tampering.

In this present case, the calibration logs on the BAC detectors showed that they gave perfectly-accurate readings every time they were tested for re-calibration. Given that their accuracy naturally deteriorates over time without regular adjustment, it’s impossible for them to give perfect readings in these regular tests. This indicates that the tests weren’t being done, and the people responsible for testing them simply falsified their logs.

So, everyone who was tested with an inaccurate device was convicted, at least in part, based on bad evidence.

Unfortunately, this is going to be a huge headache for the court system. As many as 1,000 cases, dating back to 2006, will have to be re-opened and investigated all over again. That’s a problem, since you can’t exactly go back in time and re-test a defendant’s BAC at the time they were arrested.

Many people will probably look at these facts and claim that many convicted criminals are getting off on “technicalities.” And in some way, they may be correct. I would bet that, in the majority of the cases that have to be reviewed, the defendant was actually guilty of driving with a BAC above the legal limit when they were arrested. But if the tests were inaccurate, there’s an excellent chance that a significant percentage of them, even if it’s not a majority, were innocent. Of course, it will largely be impossible to tell which is which. And this is the exact reason why all cases in which the irregularities in the testing devices create reasonable doubt as to a suspect’s guilt should be overturned.

There’s one thing that it’s essential to remember in cases like this: in all criminal cases, even “minor” ones like DUI, the burden of proof rests entirely with the prosecution. The defense attorney could sit at the defense table without saying a word, introducing a single piece of evidence, or calling a single witness. And if the jury found that the prosecution failed to provide enough evidence to convince them beyond a reasonable doubt that the defendant is guilty, the jury has no choice but to find the defendant “not guilty.” It doesn’t matter if they believe that the defendant is “probably guilty,” if they can still entertain reasonable doubt as to that fact.

In most DUI cases, a malfunctioning BAC tester, even if it’s only off by a little bit, could introduce reasonable doubt as to the defendant’s guilt, even if it’s not the only piece of evidence that led to their conviction. In those cases, the only appropriate course of action is to overturn the defendant’s conviction.

And if this means that some people convicted of DUI will escape the consequences of their crime, so be it. That may sound harsh, but the principle that everyone is presumed innocent until proven guilty is one of the basic cornerstones of our legal system. If we abandon this principle, absolutely nothing good can come of it.